1. Field of the Invention
The invention relates generally to an electrophysiological (xe2x80x9cEPxe2x80x9d) apparatus and method for providing energy to biological tissue and, more particularly, to a radio frequency (xe2x80x9cRFxe2x80x9d) ablation apparatus for controlling the flow of current through biological tissue so that the depth and continuity of ablation lesions may be controlled.
2. Description of the Related Art
The heart beat in a healthy human is controlled by the sinoatrial node (xe2x80x9cS-A nodexe2x80x9d) located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node (xe2x80x9cA-V nodexe2x80x9d) which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth, remodeling, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as xe2x80x9ccardiac arrhythmia.xe2x80x9d
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed percutaneously, a procedure in which a catheter is introduced into the patient through an artery or vein and directed to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue.
In the case of atrial fibrillation (xe2x80x9cAFxe2x80x9d), a procedure published by Cox et al. and known as the xe2x80x9cMaze procedurexe2x80x9d involves the formation of continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system providing RF ablation therapy. In this therapy, transmural ablation lesions are formed in the atria to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. In this sense transmural is meant to include lesions that pass through the atrial wall or ventricle wall from the interior surface (endocardium) to the exterior surface (epicardium).
There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is placed on the chest, back or other external location of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, electrodes introduced into the heart have different potentials and complete an electrical circuit between themselves. In both the unipolar and the bipolar methods, the current traveling between the electrodes of the catheter and between the electrodes and the backplate enters the tissue and induces a temperature rise in the tissue resulting in ablation.
During ablation, RF energy is applied to the electrodes to raise the temperature of the target tissue to a lethal, non-viable state. In general, the lethal temperature boundary between viable and non-viable tissue is between approximately 45xc2x0 C. to 55xc2x0 C. and more specifically, approximately 48xc2x0 C. Tissue heated to a temperature above 48xc2x0 C. for several seconds becomes permanently non-viable and defines the ablation volume. Tissue adjacent to the electrodes delivering RF energy is heated by resistive heating which is conducted radially outward from the electrode-tissue interface. The goal is to elevate the tissue temperature, which is generally at 37xc2x0 C., fairly uniformly to an ablation temperature above 48xc2x0 C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100xc2x0 C. In clinical applications, the target temperature is set below 70xc2x0 C. to avoid coagulum formation. Lesion size has been demonstrated to be proportional to temperature.
A basic RF ablation system for forming linear lesions includes a catheter carrying a plurality of electrodes, a backplate and an RF generator adapted to provide RF signals to the electrodes to establish bipolar or unipolar current flow. In one such ablation system, as described in U.S. Pat. No. 6,200,314, RF signals having a constant amplitude and a controllable phase angle are supplied to each electrode. A backplate is maintained at a reference voltage level in relation to the amplitude of the RF signals. The power control system controls the relative phase angles of the RF signals to establish a voltage potential between the electrodes. Current thus flows between the electrodes and between the electrodes and the backplate to produce linear lesions. In order to establish the phase difference between RF signals, the system requires a programmable logic array and a controllable frequency source. The logic array receives phase control signals from a microprocessor and controls the frequency source accordingly.
In other, less complex RF ablation systems, such as those described in U.S. Pat. Nos. 5,810,802 and 6,001,093, a controller electrically couples an indifferent electrode, i.e., backplate, and each of several electrodes to a single RF source through a network of switches. Depending on the setting of its associated switch, an electrode may be set to either an energy emitting polarity, an energy receiving polarity or neither (inactive). Using the switches, the system may be configured so that current flows between the electrodes or between the electrodes and the backplate. The system, however, does not provide for simultaneous unipolar and bipolar operation, thus lesion depth and continuity characteristics may be inadequate. Moreover, since power to all electrodes is supplied by a single source, any type of power control using temperature feedback may prove ineffective. Specifically, any power adjustments resulting from the operating conditions of one electrode, e.g., lower power due to an overheated electrode, necessarily affect the operating conditions of the remaining electrodes. This too can lead to inadequate lesion depth and continuity characteristics.
In other ablation systems having a generator with a limited number of output channels power delivery to some electrodes is not controllable. For example, as shown schematically in FIG. 13, for a six channel generator outputting power signals P1-P6 used in conjunction with a twelve band electrode catheter, every other electrode may be individually controlled while the remaining electrodes are maintained at a reference ground. Such a system has inherent problems with respect to temperature feedback power control. For example, if the temperature at electrode E2 increases above an acceptable threshold level, heat at electrode E2 is reduced by reducing the current flowing to it from adjacent electrodes E1 and E3. To accomplish this, power to electrode E1 may be shut off or reduced. If, however, the temperature at electrode E2 remains above the threshold, then the power to electrode E3 may have to be shut off or reduced. In this case little if any current flows between electrodes E1-E2 and electrodes E3-E2 and E3-E4. Thus electrodes E1 through E4 are effectively shut off due to an over-temperature condition at one electrode E2. As another example, if the temperature at electrode E4 increase above the threshold, power to electrode E3 may be shut off or reduced. This, however, affects the current path between E3 and E2. Such interdependence makes power control more difficult.
Hence, those skilled in the art have recognized a need for a multi-channel ablation system having independent power signal control capability for providing periodically fluctuating voltage potentials between electrodes to thereby induce unipolar and bipolar current flow through tissue without reliance on complex phasing circuitry. The need for a single channel ablation system having like power signal control capabilities for use in conjunction with multiple electrodes has also been recognized. The invention fulfills these needs and others.
Briefly, and in general terms, the invention is directed to sing e-channel and multi-channel ablation systems having controllable power signal capability for providing periodically fluctuating voltage potentials between electrodes to thereby establish unipolar or bipolar current flow through biological tissue.
In one aspect, the invention relates to a multi-channel system for ablating biological tissue using power signals having controllable peak-to-peak amplitudes. The system includes a catheter having a plurality of electrodes, a power generator that provides power signals to the electrodes and a processor that controls the power generator. The power generator is controlled so that during a first period of time, a first peak-to-peak amplitude signal is provided to at least one electrode defining a first electrode set and a second peak-to-peak amplitude signal is provided to at least one electrode defining a second electrode set. During the first period of time the first amplitude is greater than the second amplitude thereby establishing bipolar current flow from the first electrode set to the second electrode set. The power generator is also controlled so that during a second period of time, a third peak-to-peak amplitude signal is provided to the first electrode set and a fourth peak-to-peak amplitude signal is provided to the second electrode set. During this period of time the third amplitude is less than the fourth amplitude and current flows from the second electrode set to the first electrode set.
In a detailed aspect of the invention, the first amplitude signal and the third amplitude signal are provided by a first RF power signal while the second amplitude signal and the fourth amplitude signal are provided by a second RF power signal. The RF power signals may be continuous or duty cycled (pulsed). In a further detailed aspect the first and second RF power signals are in phase. In another detailed aspect, the processor controls the power generator so that there are a plurality of alternating first and second time periods during which any one of the first, second, third and fourth amplitudes may be varied to establish different levels of bipolar current flow between electrodes. In another detailed facet, the system includes a backplate. The power generator maintains the backplate at a reference voltage different then the peak-to-peak amplitude of any one of the first, second, third and fourth amplitudes, thereby establishing unipolar current flow between the electrodes and the backplate. In yet another detailed aspect of the invention, at least one electrode in each electrode set includes a temperature sensor or multiple temperature sensors that provide signals indicative of the temperature at the electrode to the processor. The processor converts the temperature signals to a temperature reading and compares the temperature reading to a target temperature. Based on the difference between the temperature reading and the target temperature, the processor adjusts the power provided to the electrode by the power signal.
In another aspect, the invention relates to a single channel system for ablating biological tissue using a power signal having a controllable peak-to-peak amplitude. The system includes a catheter having a plurality of electrode pairs and a power control system that establishes voltage potentials between the electrodes within an electrode pair. The power control system establishes a voltage potential between a first electrode pair during a first period of time by providing a first power signal to one of the electrodes while maintaining the other electrode at a reference potential. During a second period of time, the power control system establishes a voltage potential between a second electrode pair by providing the first power signal to one of the electrodes while maintaining the other electrode at the reference potential.
In a detailed facet of the invention, during subsequent time periods, the power control system establishes a voltage potential between each of the remaining electrode pairs by providing the first power signal to one of the electrodes while maintaining the other electrode at a reference potential. In another detailed aspect, the power control system repeatedly establishes voltage potentials between electrode pairs in sequence from the first electrode pair to the last electrode pair. In another detailed facet of the invention, the first and second electrode pairs comprise a common electrode and the first power signal is provided to the common electrode during each of the first and second time periods. In yet another detailed aspect of the invention, at least one electrode in each electrode pair includes a temperature sensor or multiple temperature sensors that provides signals indicative of the temperature at the electrode-tissue interface to the power control system. The power control system converts the temperature signals to a temperature reading and compares the temperature reading to a target temperature. Based on the difference between the temperature reading and the target temperature, the power control system adjusts the power provided to the electrode by the power signal.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings which illustrate by way of example the features of the invention.